This invention pertains to the process for preparing water-dilutable, heat-curable coating compositions which are based on aromatic polyglycidyl ethers curable with aminoplast or phenoplast curing systems.
Water-dilutability of the aromatic polyglycidyl ether based compositions can be attained by their initial reaction with polyethylene glycol to form hydroxy-containing adducts capable of later curing with conventional aminoplast or phenoplast curing systems.
The reaction of polyhydric alcohols with epoxy resins is known. In U.S. Pat. No. 2,731,444 the reaction of epoxy resins with polyhydric alcohols is taught to give compositions suitable as crosslinking reactants. There are disclosures of the reactions of epoxy resins with less than one equivalent of hydroxyl per epoxy group, with equivalent amounts of hydroxyl to epoxy group, but with incomplete reaction thereof, and of excess hydroxyl group per epoxy group. However, this patent does not teach or suggest the process of the instant invention. In every case, the epoxy resin disclosed has a higher molecular weight than the aromatic polyglycidyl ethers found operable in the process of this invention.
Although U.S. Pat. No. 2,731,444 does disclose the use of polyethylene glycols with a broad spectrum of molecular weights below, within and above the critical limits of the process of this invention, the only exemplification results in a product which is a wax-like solid insoluble, non-dilutable by water being clearly outside the critical limits of the present process.
While U.S. Pat. No. 2,731,444 teaches that the reaction products of epoxy resins and polyhydric alcohols, including polyethylene glycol, can be further reacted with various polyfunctional crosslinking reactants through epoxy or hydroxy groups thereon and that the reaction products of epoxy resins with less than equivalent amounts of polyhydric alcohols are valuable coating compositions, it does not teach or suggest the process of this invention for the preparation of water-dilutable, heat-curable coating compositions nor does it lay down the critical limits required to achieve the instant process. In fact, U.S. Pat. No. 2,731,444 teaches away from the instant process by broadly disclosing the reaction of epoxy resins and polyhydric alcohols without suggesting that water-dilutability is possible let alone a desirable objective. In fact, none of the exemplifications in the disclosure of this reference falls within the critical limits of the instant process.
The reaction of polyethylene glycol with an aromatic diepoxide is also taught in U.S. Pat. No. 3,563,943 for the preparation of oxyethylene-containing non-ionic compounds useful as emulsifiers for urethane latices. This patent teaches that the polyethylene glycol must have a molecular weight of from about 5,000 to about 10,000 and be reacted with an aromatic diepoxide in a mole ratio of polyethylene glycol to aromatic diepoxide of at least 2:1. The non-ionic emulsifiers resulting have a molecular weight of from 12,000-24,000. While the chemical reactions described here to prepare the emulsifier are in some ways similar to those employed in this invention, a much lower (400 to 800) molecular weight range of polyethylene glycol is needed to produce a hydroxy-containing material suitable for use in water dilutable coating compositions. In addition, a mole ratio of polyethylene glycol plus dihydric phenol to poly epoxide of 1:1 to 1:2 is required in the instant process.
U.S. Pat. No. 2,951,778 teaches the use of the reaction product of poly(1,2-epoxide) with monomeric ethylene glycol at mole ratios of 2:1 to 1:2 to form a flexibilizer useful in epoxy resin formulations. Such a material would be insoluble in water and of no value in the water-dilutable process of this invention.
U.S. Pat. No. 2,947,717 teaches the use of a dihydric alcohol of molecular weight less than 600, including polyethylene glycol, in combination with a poly(1,2-epoxide) and a polycarboxylic acid anhydride for the preparation of crosslinked infusible resinous products made more fluid and hence easier to use before curing by the presence of dihydric alcohol. The various ingredients are mixed in the absence of any solvent (organic or water) and heated till homogeneous at temperatures not over 80.degree. C. Heating at 80.degree.-200.degree. C effects curing and crosslinking of this system as all three components interact with one another. This system would not be water-dilutable; in fact the presence of water would render it inoperable due to premature hydrolysis of the anhydride before curing.
In U.S. Pat. No. 4,048,179, a process is described for preparing water-dilutable, heat-curable coating compositions which comprises reacting an aromatic polyglycidyl ether having a molecular weight in the range of 360 to 500 with a polyethylene glycol having an average molecular weight in the range of 400 to 800 in the presence of 0 to 20% by weight of a polyhydric phenol to form a water-dilutable, hydroxy-containing adduct; adding 5 to 50% by weight of solids of an aminoplast or phenoplast curing agent; and diluting with water to bring the total percent solids in the range of 10 to 80% by weight and to yield a stable, aqueous composition containing no particles larger in size than 0.1 micron.
In this process, the composition resides essentially in an all-aqueous medium with no organic solvent of consequence present. The chemical nature of the composition is tailored to meet the requirements for water-dilutability in an all-aqueous medium. Water-dilutability depends on the ratio of epoxy resin, to bis-phenol A to polyethylene glycol in the hydroxy-containing adduct formed in the process. For the all-aqueous medium the moles of aromatic polyglycidyl ether to combined moles of polyethylene glycol plus polyhydric phenol are in the ratio of 1.0/1.0 to 3.0/1.0, preferably 1.25/1.0 to 1.35/1.0, with the moles of polyethylene glycol to moles of polyhydric phenol in the ratio of 1.0/0.0 to 1.0/1.5.